As ‘bird flu’ in cattle raises concerns for milk safety in the US, scientists confirm that pasteurisation effectively inactivates influenza viruses

Published: 3 June 2024

A new study has confirmed that pasteurisation temperatures, if applied for industry standard times, should effectively inactivate H5N1 influenza viruses in cows’ milk and render it safe to consume.

A new study has confirmed that pasteurisation temperatures, if applied for industry standard times, should effectively inactivate H5N1 influenza viruses in cows’ milk and render it safe to consume. 

The reassuring research – a collaboration between the MRC-University of Glasgow Centre for Virus Research, the University of Edinburgh’s Roslin Institute, the Pirbright Institute and the UK Animal & Plant Health Agency, and released as a pre-print on medRxiv – confirms that pasteurised cows’ milk (the form typically sold for human consumption) should have effectively eliminated any influenza virus in it.

The study, from a team of scientists in the UK, comes in response to an outbreak of H5N1 influenza – a highly pathogenic strain of avian influenza virus or ‘bird flu’ – in American dairy cattle. This ongoing outbreak – the first time a virus of this sort has spread in cattle – led to high levels of potentially dangerous virus being shed into milk in some parts of the USA. This raised concerns about human infections from contaminated milk, including the risk that this could allow the virus to adapt to humans and cause a new pandemic.

A herd of cows. One cow, mostly black, looking towards the viewer.

The new research establishes that pasteurisation of milk can effectively kill influenza viruses. However, the study also found that without pasteurisation, milk can be a source of infectious influenza viruses, including H5N1 influenza virus. While H5N1 infections of humans are rare, when human infections do occur they have the potential to be extremely serious. Because of this, the study authors caution against the consumption of ‘raw’ cows’ milk in affected areas.

In their study, the researchers mixed a panel of influenza viruses with milk (both unpasteurised ‘raw’ milk and commercially-available whole milk) in laboratory conditions, and heated them to pasteurising temperatures of 63°C or 72°C. To gain a general understanding of the response of influenza viruses to heating they tested a panel of different viruses, including human and avian influenza A viruses (the group of viruses that H5N1 belongs to) as well as an influenza D virus (a different influenza virus that naturally infects cattle). 

Heating in a laboratory to mimic the times and temperatures reached by industrial pasteurisation effectively killed all of the influenza viruses in the study. The research team then carried out experiments in a high-containment laboratory using the highly pathogenic H5N1 ‘bird flu’ virus itself, confirming that pasteurisation could effectively kill this virus in milk, even though the remains of its genetic material could still be detected.

Although this is reassuring, the researchers also showed that if milk is not pasteurised it is an effective carrier of infectious influenza viruses, including H5N1 influenza.

The current outbreak of H5N1 avian influenza is the worst on record and since 2020 it has spread rapidly to birds around the world. There have been repeated ‘spillover’ infections of mammals, but these were generally not in close proximity to humans and the virus did not spread effectively in those species. This changed in late 2023, when H5N1 influenza infected cattle in the USA and began to spread among dairy herds. So far this outbreak has remained confined to the USA, but it is concerning, as it means that this potentially dangerous virus is now circulating in mammals who have close contact with humans.

Additionally, scientists detected the virus in the milk of infected cows, which is not a normal route of spread for influenza viruses. It appears that contaminated milk has caused further ‘spillover’ infections of animals on dairy farms, as well as of farm workers (so far, reported human cases have caused mild symptoms, with no onward transmission of the virus). Troublingly, the remains of H5N1 genetic material were also found in a large proportion of the milk sold to consumers in some affected areas of the USA, raising concerns that milk could be a route of human infection.

It has been known for a long time that cows’ milk can carry various pathogens, and so it is typically pasteurised for safe human consumption. It was predicted that pasteurisation would inactivate any H5N1 influenza virus, but prior to this outbreak there were no robust studies of the effectiveness of pasteurisation for influenza viruses in milk. This has been addressed by this study, and has also been considered by another recent article published by a separate group working the USA with milk taken from infected cows. Although there are some differences in methods between the two studies, the main findings are consistent: pasteurisation rapidly and effectively inactivates influenza viruses.

Ed Hutchinson, Senior Lecturer at the MRC-University of Glasgow Centre for Virus Research, and the corresponding author of the study, said: “Given the worries surrounding the H5N1 outbreak in dairy cattle in the United States, and the discovery of the virus in cows’ milk, we urgently needed to answer the question of whether pasteurisation made milk safe.

“We have now rigorously tested this, showing that the temperatures used in pasteurisation should rapidly inactivate all influenza viruses. However, we also found that influenza viruses remained infectious in ‘raw’ (unpasteurised) milk. Raw milk is already known to carry a variety of pathogens, and we would particularly caution people against drinking it in areas of the USA where cattle might be infected with H5N1 influenza. Human infections with H5N1 influenza viruses have the potential to be very dangerous, and they also give the virus more opportunities to adapt to transmitting between humans. Pasteurisation of milk in affected areas is a good way to minimise these risks.

“Our study looked at virus in milk, but we have not yet assessed if it could remain infectious in unpasteurised cheeses, raw milk yoghurt or other unpasteurised dairy products. More generally, more work is needed to understand how this virus is spreading among cattle in the USA and, given the global distribution of H5N1 in birds, whether similar outbreaks could take place in farmed animals elsewhere in the world. Virologists are currently working urgently to find answers to these questions.”

Dr Jenna Schafers, a PhD student at the University of Edinburgh Roslin Institute and a joint lead author of the study, said: “This team effort from a consortium of researchers across the UK has shown that pasteurisation temperatures rapidly inactivate H5N1 influenza viruses, which is reassuring. Our ability to respond quickly to this urgent question highlights the importance of collaboration across organisations to rapidly respond to novel issues, which is of particular importance in the post-pandemic era.”

Prof Ian Brown at the Pirbright Institute, an author on the study who is leading a UK research consortium which was part of the study team working on animal influenza, including in dairy cattle, commented: “This reactive UK work to an emerging threat underlines the strength of the UK science community. Whilst this infection with high pathogenicity avian influenza virus in dairy cattle is confined to the USA, it is important that we support the global efforts to better understand the disease, the risks it presents to the public and its control. This study on pasteurisation provides important information that underpins disease preparedness and response beyond the USA should it be required.”

The study, ‘Pasteurisation temperatures effectively inactivate influenza viruses in milk,’ is a pre-print published on medRxiv and has not yet been through peer review. The study was funded by the MRC, the Biotechnology and Biological Sciences Research Council (BBSRC); the Department for Environment, Food and Rural Affairs (Defra, UK); Wellcome; and the devolved Scottish and Welsh governments.

First published: 3 June 2024

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